1. Field of the Invention
This invention relates in general to the field of marine navigational devices and in particular to improved navigational apparatus which interfaces with a vessel's hyperbolic radionavigational receiver and its auto pilot.
2. Description of the Prior Art
Most modern-day sea-going vessels use an aid known as a Loran (long range aid to navigation) as a means to navigate the vessel from the departure point to its destination. The first operational Loran system, the Loran A, was developed during the Second World War at the Massachusetts Institute of Technology. While the initial implementation of the system was to fulfill wartime needs, it is now used by civilians as well as the military. The Loran system utilizes land-based transmitters in conjunction with onboard receivers. Each of at least two transmitters, transmits pulsed, low frequency (100 kHz) radio waves. At any given vessel position, the location of that position is a function of the distance between the vessel and each of the two transmitters. The difference in distance is in turn a function of the time difference between signals from each transmitter to the vessel. The difference in distance will lie along a hyperbolic line comprising the locus of all positions having exactly the same time difference between the times of arrival of the signals from the transmitters to the vessel. The hyperbolic line is uniformly referred to as a line of position--LOP. Since only one LOP has been established, the vessel's position has not yet been fixed. It is only known that the vessel's position is somewhere along this particular LOP.
To fully fix the vessel's position, another set of transmitters is used. In the same manner as described above, a second LOP is established. The vessel's exact position is at the intersection of the two LOPs. In practice, three, rather than four, transmitters are used. One of the transmitters being common to each set of two transmitters.
The United States Coast Guard is currently responsible for operation of the Loran C system. The Loran C is a much improved version of its predecessor the Loran A system. In 1974, eight Loran C chains were in operation, which included the use of thirty-one transmitting stations in providing coverage for over sixteen million square miles. The coverage is ultimately planned to include the entire United States Coastal Confluence Zone and a large portion of the northern hemisphere.
As mentioned, the Loran C system is used by navigators having onboard receivers specifically designed to receive the signals transmitted at approximately 100 kHz. There are numerous, very sohpisticated and fairly inexpensive Loran C receivers on today's market. In general, the receivers will visually show, by use of a light-emitting diode display, the LOPs of a particular position of a vessel. Having this information, the navigator merely refers to LOP charts to fix his position. The receivers will also convert from a LOP position to a latitude-longitude position. Perhaps one of the more important features of most if not all the receivers is the steer function. The steer function allows a navigator to steer a course along any Loran C chart line. A steer-computed line function allows a navigator to steer along any straight line course. This latter function enables a navigator to steer along the straight line between the point of origin and the desired point of destination. Again, in general, this is accomplished by first determining the vessel's position in terms of intersecting LOPs. Then, the destination LOP's are determined from an appropriate chart. These data are then input into the receiver and the navigator turns the vessel and rudder in an approximate direction to the destination. The receiver will then visually display a vertical bar or bars of light which will be centered along the horizontal display when the vessel is exactly on course. The vertical bar will move in incremental steps to the left or the right depending upon whether the vessel is off course to the left or the right, respectively. The further the vessel is off course, the further the vertical bar will be to the left or the right on the display panel. Typically, one increment represents an off course error of 50 feet. Thus, if the bar of light is three increments to the right, the vessel is 150 feet off course to the right. To put the vessel back on course, it is steered to the left until such time as the bar of light is again at the center of the display console. In actual practice, steering a course by use of the Loran C display results in the vessel making a series of large "S" curves as the vessel tracks back and forth across the computed steer line.
It should be noted that steering a computed course is totally independent of winds, currents and compass errors. In fact, a compass reading is not even needed. This is not, however, to say that a Loran C receiver replaces a compass. It is supplemental thereto.
An auto pilot is another navigational device which has long been known to be a steering aid to ship's captains. Auto pilots, of course, automatically steer a vessel along a desired magnetic bearing. Such devices, however, are not as accurate as steering along a Loran C computed steer line. This is because a magnetic bearing does not account for a vessel's drifting due to currents, wind or tide. Still, for many navigational purposes, an auto pilot is a very useful device.
A very sophisticated system is obtained when a Loran C receiver is integrated with an auto pilot. Instead of a magnetic input, however, the Loran C computed steer line comprises the directon that the auto pilot maintains. With such a system, a vessel's captain need only be concerned with avoiding unforeseen obstacles such as other vessels, low water, uncharted islands, localized thunderstorms, and the like. The advantages of such a system are immense. Human error is virtually eliminated. Missing a point of destination is all but impossible.
My previous invention entitled "Marine Navigational Aid", Ser. No. 06/209,281 now U.S. Pat. No. 4,383,259, filed Nov. 24, 1980, describes apparatus which interfaced with the then state of the art Loran C receivers and auto pilots. That apparatus was intended to provide the benefits of a combined receiver and auto pilot without the need to replace either the Loran C receiver or the auto pilot with which a vessel was already equipped.
Since the disclosure of the interface in my previous patent application, the only significant change to today's Loran C receivers has been the installation of a standard NMEA output plug on the receivers which electronically outputs the on course, off course, distance off course left and right information previously shown on the face of the receiver by LEDs. Thus, the steer function information of the Loran C receiver is now electronically output and visually displayed.
The present invention is intended to utilize the NMEA output plug in interfacing between the Loran receiver and the auto pilot and to provide increased performance over the previously disclosed interface and to allow the use of the auto pilot in virtually its normal mode of operation.